Sub-30 nm lithography with near-field scanning optical microscope combined with femtosecond laser

We report direct laser writing of lithography patterns with a feature width of 20±5 nm on thin photoresist film by combining a double-frequency femtosecond laser and a near-field scanning optical microscope. The obtained feature size is much smaller than the laser wavelength () and the aperture diameter (d) with a resolution of /20 and d/2, respectively. The lithography patterns were analyzed with an atomic force microscope and a scanning electron microscope. The effects of laser energy and writing speed on the feature size were investigated. The underlying physical mechanism was also discussed. PACS 07.79.Fc; 07.79.Lh; 42.70.Jk; 68.37.Hk; 81.16.Nd     

Field enhancement analysis of an apertureless near field scanning optical microscope probe with finite element method

Plasmonic field enhancement in a fully coated dielectric near field scanning optical microscope(NSOM) probe under radial polarization illumination is analyzed using an axially symmetric three-dimensional(3D) finite element method(FEM)model.The enhancement factor strongly depends on the illumination spot size,taper angle of the probe,and the metal film thickness.The tolerance of the alignment angle is investigated.Probe designs with different metal coatings and their enhancement performance are studied as well.The nanometric spot size at the tip apex and high field enhancement of the apertureless NSOM probe have important potential application in semiconductor metrology.     

Single-beam analysis of damaged beams: Comparison using Euler–Bernoulli and Timoshenko beam theory

A new general formulation that is applicable to the damaged, linear elastic structures ‘unified framework’ is used to obtain analytical expressions for natural frequencies and mode shapes. The term mode shapes is used to mean the displacement modes, the section rotation modes, the sectional bending strain modes and sectional shear strain modes. The formulation is applicable to damaged elastic self-adjoint systems. The formulation has two unique aspects: First, the theory is mathematically rigorous since no assumptions are made regarding the physical behavior at a damage location, therefore there is no need to substitute the damage with a hypothetical elastic element such as a spring. Since the beam is not divided at the damage location, rather than an 8 by 8, only a 4 by 4 matrix is solved to obtain the natural frequencies and mode shapes. Second, the inertia effects due to damage which have till now been neglected by researchers are accounted for. The formulation uses a geometric damage model, perturbation of mode shapes and natural frequencies, and a modal superposition technique to obtain and solve the governing differential equation. Timoshenko beam theory is then taken as an example, and its results are compared with results using Euler–Bernoulli beam theory and finite element models. The range of applicability of the two theories is ascertained for damage characteristics such as depth and extent of damage and beam characteristics such as slenderness ratio and Poisson?s ratio. The paper considers rectangular notch like non-propagating damage as an example of the damage.     

Resolution enhancement of pump–probe microscope with an inverse-annular filter

Optical Review - Optical pump–probe microscopy can provide images by detecting changes in probe light intensity induced by stimulated emission, photoinduced absorbance change, or...     

Selective photocurrent generation in the transparent wavelength range of a semiconductor photovoltaic device using a phonon-assisted optical near-field process

In this paper, we propose a novel photovoltaic device using P3HT and ZnO as test materials for the p-type and n-type semiconductors, respectively. To fabricate an electrode of this device, Ag was deposited on a P3HT film by RF-sputtering under light illumination (wavelength λ ₀=660 nm) while reversely biasing the P3HT/ZnO pn-junction. As a result, a unique granular Ag film was formed, which originated from a phonon-assisted process induced by an optical near-field in a self-organized manner. The fabricated device generated a photocurrent even though the incident light wavelength was as long as 670 nm, which is longer than the long-wavelength cutoff λ _c (=570 nm) of the P3HT. The photocurrent was generated in a wavelength-selective manner, showing a maximum at the incident light wavelength of 620 nm, which was shorter than λ ₀ because of the Stark effect brought about by the reverse bias DC electric field applied during the Ag deposition.     

Vibration of vehicle–pavement coupled system based on a Timoshenko beam on a nonlinear foundation

This paper focuses on the coupled nonlinear vibration of vehicle–pavement system. The pavement is modeled as a Timoshenko beam resting on a six-parameter foundation. The vehicle is simplified as a spring–mass–damper oscillator. For the first time, the dynamic response of vehicle–pavement coupled system is studied by modeling the pavement as a Timoshenko beam resting on a nonlinear foundation. Consequently, the shear effects and the rotational inertia of the pavement are included in the modeling process. The pavement model is assumed to be a linear-plus-cubic Pasternak-type foundation. Furthermore, the convergent Galerkin truncation is used to obtain approximate solutions to the coupled vibratory response of the vehicle–pavement coupled system. The dynamic responses of the vehicle–pavement system with the asphalt pavement on soft soil foundation are investigated via the numerical examples. The numerical results show that the calculation for the coupled vibratory response needs high-order modes. Moreover, the coupling effects between the pavement and the vehicle are numerically examined by using the convergent modal truncation. The physical parameters of the vehicle–pavement system such as the shear modulus are compared for determining their influences on the coupled vibratory response.     

Enhanced near-field distribution inside substrates mediated with gold particle: optical vortex and bifurcation

We present new results for laser excited semiconductor crystals probed by ultrashort x-ray pulses diffracted by the crystal lattice. First we discuss acoustical phonons of semiconductors, i.e. indium antimonite and germanium, and then we describe the temporal evolution of the laser-induced metal–semiconductor phase transition of samarium sulfide.     

Dynamic generation of plasmonic Moiré fringes using phase-engineered optical vortex beam

Dynamic generation of plasmonic Moiré fringes using a phase engineered optical vortex (OV) beam is experimentally demonstrated. Owing to the unique helical phase carried by an OV beam, the initial phase of surface plasmon polaritons (SPPs) emanating from a metallic grating can be adjusted dynamically by changing the phase hologram displayed on a spatial light modulator. Plasmonic Moiré fringes are readily achieved by overlapping two SPP standing waves with certain angular misalignment, excited by the positive and negative topological charge components, respectively, of a cogwheel-like OV beam. The near-field scanning optical microscopy measurement result of SPP distributions has shown a good agreement with the numerical predictions.     

Evaluation of the dynamic range and spatial resolution of nonadiabatic optical near-field lithography through fabrication of Fresnel zone plates

Fresnel zone plates (FZPs) were fabricated in order to evaluate the performance of nonadiabatic photolithography by exploiting the localized nature of optical near fields. This novel photolithography scheme could realize FZPs with structures smaller than the wavelength of the light source used for exposure. The FZP for 325-nm-wavelength UV light could focus the incident light to a spot size of 590 nm. An FZP for focusing soft X-rays was also fabricated and, compared to conventional adiabatic photolithography, showed higher-contrast zones over the whole area of the FZP. This method exhibits a high dynamic range and good spatial resolution, and it was free from artifacts due to the interference of the residual propagating exposure light transmitted through the aperture of the photomask.     

Microbending loss and refractive index changes induced by transient thermal loading in double-coated optical fibers with thermal contact resistance

The transient microbending loss and refractive index changes in a double-coated optical fiber subjected to thermal loading with stress-dependent interlayer thermal contact resistance is investigated. The effects of interlayer thermal resistance on the transient microbending loss and refractive index changes of the optical fiber are analyzed and discussed. The results show that the stress-dependent interlayer thermal contact resistance will increase the lateral pressure induced by the transient thermal loading in the double-coated optical fiber and, thus, the microbending loss. Similarly, the interlayer thermal contact resistance will increase the transient thermal loading induced refractive index changes in the beginning of loading.     

Effects of gamma radiation on hard dental tissues of albino rats using scanning electron microscope – Part 1

In the present study, 40 adult male albino rats were used to study the effect of gamma radiation on the hard dental tissues (enamel surface, dentinal tubules and the cementum surface). The rats were irradiated at 0.2, 0.5, 1.0, 2.0, 4.0 and 6.0 Gy gamma doses. The effects of irradiated hard dental tissues samples were investigated using a scanning electron microscope. For doses up to 0.5 Gy, there was no evidence of the existence of cracks on the enamel surface. With 1 Gy irradiation dose, cracks were clearly observed with localized erosive areas. At 2 Gy irradiation dose, the enamel showed morphological alterations as disturbed prismatic and interprismatic areas. An increase in dentinal tubules diameter and a contemporary inter-tubular dentine volume decrease were observed with higher irradiation dose. Concerning cementum, low doses,<0.5 Gy, showed surface irregularities and with increase in the irradiation dose to≥1 Gy, noticeable surface irregularities and erosive areas with decrease in Sharpey's fiber sites were observed. These observations could shed light on the hazardous effects of irradiation fields to the functioning of the human teeth.     

A scalable quantum computer with ultranarrow optical transition of ultracold neutral atoms in an optical lattice

We propose a new quantum-computing scheme using ultracold neutral ytterbium atoms in an optical lattice, especially in a monolayer of three-dimensional optical lattice. The nuclear Zeeman sublevels define a qubit. This choice avoids the natural phase evolution due to the magnetic dipole interaction between qubits. The Zeeman sublevels with large magnetic moments in the long-lived metastable state are also exploited to address individual atoms and to construct a controlled-multiqubit gate. Estimated parameters required for this scheme show that this proposal is scalable and experimentally feasible.     

Raman spectroscopic study of femtosecond laser-induced phase transformation associated with ripple formation on single-crystal SiC

Raman spectroscopy was performed to investigate microscopic structural changes associated with a ripple structure formation initiated by femtosecond laser irradiation on the surface of single-crystal silicon carbide. The amorphous phases of silicon carbide, silicon, and carbon were observed. The intensity ratio between amorphous silicon carbide and silicon changed discretely at the boundary between fine and coarse ripples. The physical processes responsible for the formation of the ripple structure are discussed.     

Evidences dominating the formation of ZnO nanostructures via in-situ study in an environmental scanning electron microscope

In order to well understand the growth mechanism of the diverse morphology of the ZnO nanostructures, in situ analysis of the formation of different ZnO nanostructures, such as nanowires, nanocombs, and nanosheets, has been conducted in an environmental scanning electron microscope (ESEM). It is found that both nanocombs and nanosheets grew in two-stage heating processes on parent nanowires. The difference is that the nanocombs were synthesized in extremely high pressure of zinc vapor via a self-catalyzed vapor-liquid-solid process, while the ZnO nanosheets were grown in relatively low pressure of zinc vapor. All the growth processes were revealed in real time imaging. It is demonstrated that the change in the growth environments can influence the thickness of the ZnO polycrystalline surface of the zinc powder, which alters the pressure of the zinc vapor and in turn determines the morphology of the final nanostructures.     

The rapid prototyping of textured amorphous surfaces for the graphoepitaxial deposition of CdTe films using focused ion beam lithography

Cadmium telluride films deposited on amorphous substrates exhibit a grain structure characterized by [111]-oriented grains, but where the in-plane grain orientation is randomized due to the absence of epitaxy. Here, we explore the viability of promoting an in-plane grain alignment through graphoepitaxy. Fifteen different substrate surface textures were fabricated using focused ion beam lithography. This approach allows for the side-by-side deposition of surface textures where both the areal extent and depth of the surface features are varied in a systematic manner. CdTe films deposited overtop these textures show grain structures with dramatic variations, revealing that particular length scales have the most pronounced effect on the grain structure.     

Microlensing as a probe of the Galactic structure: 20 years of microlensing optical depth studies

Microlensing is now a very popular observational astronomical technique. The investigations accessible through this effect range from the dark matter problem to the search for extra-solar planets. In this review, the techniques to search for microlensing effects and to determine optical depths through the monitoring of large samples of stars will be described. The consequences of the published results on the knowledge of the Milky-Way structure and its dark matter component will be discussed. The difficulties and limitations of the ongoing programs and the perspectives of the microlensing optical depth technique as a probe of the Galaxy structure will also be detailed.     

Enhancement of resolution and quality of nano-hole structure on GaN substrates using the second-harmonic beam of near-infrared femtosecond laser

By using a second harmonic of near infrared femtosecond (fs) laser (λ=387 nm, 150 fs) with high NA objective lens, fabrication resolution has been greatly improved in nano-fabrication of wide band-gap semiconductor gallium nitride (GaN). We have carried out a wet-chemical-assisted fs laser ablation method, in which the laser beam is focused onto a single-crystal GaN substrate immersed in a concentrated hydrochloric (HCl) acid solution. A two-step processing involving irradiation with a fs laser beam in air followed by wet chemical treatment is also performed for comparison. In the wet-chemical-assisted ablation, theoretical diameters of ablation craters are calculated as a function of pulse energy by assuming that the reaction is based on two-photon absorption. In lower energy, the calculated curve is close to the experimental value, while the actual measured diameters in the region of higher energy are larger than calculated values. In the condition of the highest fabrication resolution, we obtained ablation craters smaller than 200 nm at full width at half maximum. We have also demonstrated the fabrication of two-dimensional (2D) periodic nanostructures on surface of a GaN substrate using the second harmonic single fs-laser pulse. Uniform ablation craters with the size as small as 410 nm in diameter are arranged with a periodicity of 1 μm. Such structures are applicable to 2D photonic crystals which improve the light extraction efficiency for blue LEDs in the near future.     

The effect of cesium metal clusters on the optical properties of cesium iodide thin films

Cesium metal clusters strongly affect the optical properties of cesium iodide thin films. The metal clusters are formed during film formation by thermal evaporation. The cesium cluster of 30–40 nm in the matrix of cesium iodide insulating thin films results in Surface Plasmon Resonance (SPR). The peak position of these SPR peaks showed a red shift. This was shown to be due to changes in the dielectric constant of CsI resulting from the strains in the films caused by the metal clusters themselves.     

Molecular dynamics simulations of nanopore processing in a graphene sheet by using gas cluster ion beam

The formation of a nanopore in a graphene sheet by collision with an argon cluster is simulated using molecular dynamics method. The number of removed carbon atoms and the size of the nanopore are obtained as a function of the kinetic energy of the cluster. In contrast to nanosculpting with a monomer ion beam, the size of the nanopore that is created by one shot of the cluster varies because of the variety of atom configuration. However, the mean size of the nanopore can be controlled over a wide range only by changing the kinetic energy of the cluster. This implies that the cleaning and processing of the graphene sheet may be realized simultaneously by changing the acceleration energy of the cluster.